Self-guiding light in layered nonlinear media

We study the propagation of intense optical beams in layered Kerr media. With appropriate shapes, beams with a power close to the self-focusing threshold are shown to propagate over long distances as quasi-stationary waveguides in cubic media supporting a periodic nonlinear refractive index.     

The geometric phase in nonlinear frequency conversion

The geometric phase of light has been demonstrated in various platforms of the linear optical regime, raising interest both for fundamental science as well as applications, such as flat optical elements. Recently, the concept of geometric phases has been extended to nonlinear optics, following advances in engineering both bulk nonlinear photonic crystals and nonlinear metasurfaces. These new technologies offer a great promise of applications for nonlinear manipulation of light. In this review, we cover the recent theoretical and experimental advances in the field of geometric phases accompanying nonlinear frequency conversion. We first consider the case of bulk nonlinear photonic crystals, in which the interaction between propagating waves is quasi-phase-matched, with an engineerable geometric phase accumulated by the light. Nonlinear photonic crystals can offer efficient and robust frequency conversion in both the linearized and fully-nonlinear regimes of interaction, and allow for several applications including adiabatic mode conversion, electromagnetic nonreciprocity and novel topological effects for light. We then cover the rapidly-growing field of nonlinear Pancharatnam-Berry metasurfaces, which allow the simultaneous nonlinear generation and shaping of light by using ultrathin optical elements with subwavelength phase and amplitude resolution. We discuss the macroscopic selection rules that depend on the rotational symmetry of the constituent meta-atoms, the order of the harmonic generations, and the change in circular polarization. Continuous geometric phase gradients allow the steering of light beams and shaping of their spatial modes. More complex designs perform nonlinear imaging and multiplex nonlinear holograms, where the functionality is varied according to the generated harmonic order and polarization. Recent advancements in the fabrication of three dimensional nonlinear photonic crystals, as well as the pursuit of quantum light sources based on nonlinear metasurfaces, offer exciting new possibilities for novel nonlinear optical applications based on geometric phases.     

Optimizing non-resonant frequency conversion in periodically poled media

Non-resonant frequency conversion into the blue, green, orange, and red spectral regions is reported. Fundamental light sources were continuous-wave non-planar monolithic single-mode ring Nd:YAG lasers as well as a standing-wave multi-mode Nd:YAG laser. Periodically poled KTiOPO₄ was employed as the nonlinear medium, but the considerations could also be applied to other periodically poled materials. A multi-pass scheme resulted in a normalized conversion efficiency as high as 27.2 %W^-1 for frequency doubling in the small-signal regime at 1064 nm. PACS 42.65.Ky; 42.79.Nv; 42.70.Mp     

Photonic quasicrystals for nonlinear optical frequency conversion

We present a general method for the design of 2-dimensional nonlinear photonic quasicrystals that can be utilized for the simultaneous phase matching of arbitrary optical frequency-conversion processes. The proposed scheme--based on the generalized dual-grid method that is used for constructing tiling models of quasicrystals--gives complete design flexibility, removing any constraints imposed by previous approaches. As an example we demonstrate the design of a color fan--a nonlinear photonic quasicrystal whose input is a single wave at frequency omega and whose output consists of the second, third, and fourth harmonics of omega, each in a different spatial direction.     

基于非线性频率变换的激光技术实验

利用KTP晶体和BBO晶体对激光二极管(LD)泵浦Nd:YVO4晶体声光调Q产生的1 064 nm红外光进行二倍频与四倍频, 实现了高效的紫外激光输出. 采用腔外倍频技术可使学生看到倍频前后的激光变换过程.     

Efficient nonlinear frequency conversion in an all-resonant double- lambda system

We demonstrate efficient, pulsed, gas-phase, nonlinear frequency conversion in a quadruply resonant, double- Lambda system and, simultaneously, verify theoretical predictions of Rabi-frequency matching unique to absorbing nonlinear media. This system is used to up-convert ultraviolet light at 233 nm to the vacuum ultraviolet at 186 nm in atomic Pb vapor with small-signal conversion efficiencies exceeding 30% and with modest atomic density-length (NL) products (scale 10(14) cm(-2)) and optical power densities (10-100 kW/cm(2)).     

Combined frequency conversion and pulse compression in nonlinear tapered waveguides

We suggest an application of pump-degenerate four-wave mixing process in tapered waveguides for generation of ultrashort pulses with central frequency tunable over the material transparency range. Our method can produce strongly compressed frequency-converted pulses in presence of group-velocity mismatch and group-velocity dispersion. Additionally, the proposed technique does not require pulse phase synchronization and effectively operates for strongly chirped pump pulses, thus enabling the use of longer nonlinear media for high conversion efficiency.     

Optimal poling of nonlinear photonic crystals for frequency conversion

Nonlinear photonic crystals formed by two-dimensional periodic poling of the quadratic susceptibility chi(2) can support quasi-phase-matched harmonic generation. The frequency-conversion efficiency depends on the photonic crystal's poling pattern through certain Fourier coefficients of the poling function. A procedure is described for finding those poling patterns that are most efficient for any specific quasi-phase-matched frequency-conversion scheme.     

Polarization-free second-order nonlinear frequency conversion using the optical superlattice

We propose a universal and practical scheme to realize polarization-free second-order nonlinear frequency-conversion processes, including sum-frequency generation, difference-frequency generation, and optical parametric amplification. This scheme is based on the optical superlattice with a noncritical phase-matching condition, which is suitable for optical integration. A chirped dual-period structure is proposed to ensure the efficiency and stability of the conversions.     

Resonance scattering and generation of oscillations by channeling nonlinear layered media

For the first time within the framework of a self-consistent analysis of the process of resonant scattering and generation of oscillations by a channeling nonlinear layer it is shown that the imaginary part of the nonlinear component of the permittivity, induced by the intensity of the incident field, takes both positive and negative values along the height of the layer. The variation of this quantity characterizes the loss of energy in the nonlinear medium which is spent for the generation of the electromagnetic field at the higher harmonics. The consideration of a weak excitation field at a multiple frequency leads merely to an increase of the portion of generated energy.     

Geometric phase opens new frontiers in nonlinear frequency conversion of light

Narrow-bandgap materials possess the intriguing optical-electric properties and unique structures,which can be widely applied in the field of photonics,energy optoelectronic sensing and biomedicine,etc.Nowadays,the researches on nonlinear optical properties of narrow-bandgap materials have attracted extensive attention worldwide.In this paper,we review the progress of narrow-bandgap materials from many aspects,such as background,nonlinear optical properties,energy band structure,methods of preparation,and applications.These materials have obvious nonlinear optical characteristics and the interaction with the short pulse laser excitation shows the extremely strong nonlinear absorption characteristics,which leads to the optical limiting or saturable absorption related to Pauli blocking and excited state absorption.Especially,some of these novel narrow-bandgap materials have been utilized for the generation of ultrashort pulse that covers the range from the visible to mid-infrared wavelength regions.Hence,the study on these materials paves a new way for the advancement of optoelctronics devices.     

飞秒激光非线性频率转换的自适应优化过程

使用闭环控制的自适应控制光学系统对飞秒激光的波前进行调控,可以有效改善激光与物质相互作用中由于相位畸变或光路偏差导致的效率下降。利用可编程纯相位的液晶空间光调制器结合模拟退火算法设计的自适应优化系统,在BBO晶体的飞秒倍频过程中实现了对相位畸变和光路偏差导致倍频效率下降的自动补偿,从而实现飞秒倍频效率的最优化。     

A high-order numerical method for the nonlinear Helmholtz equation in multidimensional layered media

We present a novel computational methodology for solving the scalar nonlinear Helmholtz equation (NLH) that governs the propagation of laser light in Kerr dielectrics.     

On the enhancement of the nonlinear frequency transformation in periodic media

It is theoretically shown that an enhancement of the nonlinear frequency transformation in a periodic medium occurs under definite conditions if it is accompanied by light diffraction in this medium. The enhancement of the phase-matched frequency transformation occurs if the harmonic frequency ω approaches the stop band frequency ω_e. In the nondepleted pump approximation the corresponding increase of the harmonic intensity is proportional to ω_e(ω?ω_e)^?1 and is restricted by a quantity proportional to the fourth power of the sample thickness. The formulas for the enhancement of second harmonic generation are presented.     

Lateral patterning of nonlinear frequency conversion with transversely varying quasi-phase-matching gratings

We demonstrate control of the nonlinear conversion across a beam profile by using periodically poled lithium niobate with a laterally nonuniform quasi-phase-matching grating. As a representative experiment, generation of a flat-top second-harmonic beam is demonstrated.     

Ultrashort pulse propagation and nonlinear frequency conversion in superconducting and magnetic photonic crystal

We study the propagation and second harmonic generation of ultrashort pulse in nonlinear photonic crystal using a combination of Fourier transform and transfer matrix method. The focus is on the reflected and transmitted output of fundamental pump pulse and second harmonic pulse in frequency and time domains, the temperature dependence of the reflection and transmission spectra where the superconducting transition frequency is close to the magnetic resonance. Interesting features include output pump and second harmonic pulses that can be strongly modulated with the transmitted pulses being delayed by slow light effect.     

Origin and suppression of back conversion in a phasematched nonlinear frequency down-conversion process

Back conversion is an intrinsic phenomenon in nonlinear frequency down-conversion processes. However, the physical reason for its occurrence is not well understood. Here, we theoretically reveal that back conversion is the result of a π-phase jump associated with the depletion of one interacting wave. By suppressing the idler phase jump through a deliberate crystal absorption, the back conversion can be inhabited, thus enhancing the conversion efficiency from the pump to the signal. The results presented in this Letter will further the understanding of nonlinear parametric processes and pave the way toward the design of highly efficient down-conversion systems.     

Optical harmonic generation and laser light frequency mixing processes in nonlinear media

The essential results achieved during the ten years since the coming of lasers, in the fields of second, third and higher harmonic generation in reflected and transmitted, continuous and pulsed laser light, frequency mixing processes in various materials, and nonlinear optical properties of liquids, gases, crystals and other materials are reported concisely from the available literature. Stress is laid on the newer and more stimulating experimental results, and on their relation with other nonlinear phenomena.Invited review article given on 6 April 1970, at the IVth Polish Conference on Radiospectroscopy and Quantum Electronics in Pozna. This review covers only such papers as were directly accessible to the author.